A short stratigraphic interval near Bulin in western Hunan (China) yields multiple specimens of the ~514 Ma old oryctocarine trilobite Oryctocarella duyunensis. Size data obtained from these specimens indicates that, from meraspid degree 1 onwards, degrees represent successive instars. Meraspid growth persisted until a terminal stage was reached, providing the first example of determinate growth in trilobites and, notably, in an early Cambrian species. The sample contains three varieties of such terminal stages, recognized as holaspids, with 9, 10, or 11 thoracic segments respectively. During the meraspid phase growth rates were not constant in this species. The pattern of growth seen in the Bulin assemblage differs modestly from that reported in the same species from two other localities, attesting to microevolutionary variation in developmental patterns among these collections.

Data from:
Dai T., Hughes N.C., Zhang X.-L, Fusco G. (2021). Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis. Paleobiology.
[Dataset 1 and 2 have also been used in Dai et al.(2021). Development of the early Cambrian oryctocephalid trilobite Oryctocarella duyunensis from western Hunan, China. Journal of Paleontology.]

DATASET 1
Dataset 1 comprises 968 specimens that permitted measurement of their sagittal dorsal length and the number of thoracic segments

Legend of dataset 1
NTH    Number of THoracic segments
BL    Body Length (in millimeters)
lnBL    Natural logarithm of Body Length

DATASET 2
Dataset 2 is a subset of dataset 1 which comprises 643 specimens where, in addition, segment number in the pygidia could also be counted with confidence

Legend of dataset 2
NTH    Number of THoracic segments
NPY    Number of PYgidial segments
BL    Body Length (in millimeters)
lnBL    Natural logarithm of Body Length

DATASET 3
Dataset 3 is a subset of dataset 2 which comprises 337 specimens in which landmark-based measurements could be taken for an array of characters

Legend of dataset 3
Stage    Developmental stage (instar) assignment based on the number of thoracic segments (see text)
morph    Morph description based on the number of thoracic segments (first figure) and the number of pygidial segments (second figure)
NTH    Number of THoracic segments
NPY    Number of PYgidial segments
CCS    Cephalic Centroid Size
CEL    CEphalic Length    
TRL    TRunk Length     
PYL    PYgidial Length     
PCj    Distance of the Posterior boundary of Cephalic segment j from cephalon anterior boundary (PC0= frontal piece; PC6=CEL)   [corresponds to the cumulative segment length of the cephalon until cephalic segment j included]
PTi    Distance of the Posterior boundary of Trunk Segment i from cephalon/trunk boundary  [corresponds to the cumulative segment length of the trunk until trunk segment i included]

All measures are in millimeters

Colors refer to segmental units of different body parts:
Olive green    Frontal piece
Green             Cephalon to the exclusion of the frontal piece
Dark blue       Thorax
Blue               Pygidium to the exclusion of the terminal piece
Purple            Terminal piece

Note that in Fig. 1 junctions within the thorax are shown by pink diamonds, and those within the pygidium, and its posterior, in orange.